The present invention relates to a filtering device and a pressure sensitive valve therefor. Such a filtering device can be advantageously used in the medical treatment of blood, e.g., hemofiltration and plasmapheresis, which are methods of thickening blood and consequently a pressure drop occurs.
Hemofiltration refers to a process wherein the blood fluid is withdrawn from the patient together with toxins which could not be removed by means of the normal renal function. A quantity of liquid on the order of magnitude of 20 liters is withdrawn from the patient. It is desirable to supply back to the patient substantially the same quantity of replacement liquid, since a certain reduction in weight is not usually desired.
Plasmapheresis refers to a process wherein only the blood plasma is withdrawn from the blood donor, and the blood cells and similar larger molecules together with a small portion of the blood plasma are returned. In this process, a certain quantity of replacement liquid also may be supplied to the patient.
Both hemofiltration and plasmapheresis take place appropriately with the help of membrane filtration in devices which resemble normal membrane dialyzers. Usually, however, more permeable membranes and higher pressures are used than with dialysis. Moreover, no dialysis liquid is employed on the side of the membrane remote from the blood.
Normally, separate filtering devices are used for processes of the above-mentioned type which are thus similar to normal membrane dialyzers. Tests have also been carried out with filtering devices coupled in paralled or in series. One such device comprises an inlet and an outlet for the liquid filtered and an outlet for the filtrate, the filtration being arranged to take place in two chambers connected in series, which chambers consist at least partially of membrane material through which the filtration is taking place. A series connection in combination with an increase in pressure drop is especially suitable, since the latter gives an increase in the amount of filtrate. However, if the pressure drop across the filtering device is too great, problems may arise. For example, the blood may be damaged through hemolysis.
Experiments have also been carried out with chambers connected in series and in parallel arranged in one and the same casing, see, for example, U.S. Pat. No. 4,038,190. Difficulties may be experienced, however, with the designs described in this patent, if, in particular for hemofiltration or plasmapheresis, a sufficiently high pressure is to be achieved without the risk of too high a pressure when, for example, the filter becomes choked.